There are inkjet image forming apparatuses having an inkjet image forming device such as an inkjet recording head to eject ink droplets toward a recording material such as a paper sheet (hereinafter referred to as a recording sheet). In such inkjet image forming apparatuses, ink images on a recording material are not completely dried soon after the images are formed on the recording material. Therefore, the inkjet image forming apparatuses typically use a face-up discharging method in which a recording sheet bearing an image thereon is discharged from the image forming apparatuses in a face-up manner such that the image faces upward.
In contrast, there are inkjet image forming apparatuses using a face-down discharging method in which a recording sheet bearing an image thereon is discharged from the image forming apparatuses in a face-down manner such that the image faces downward so that the recording sheets stacked on a stacker can be collated or the layout of devices of the image forming apparatuses can be optimized.
Further, inkjet image forming apparatuses have a big problem in that when an ink image is formed on a paper sheet serving as a recording sheet, the paper sheet is curled by water included in the ink of the ink image. In this regard, it is well known that as water included in an ink image is penetrated into the paper sheet, the degree of curl of the paper sheet decreases.
However, there has been no proposal to effectively reduce curling of paper sheets to orderly stack paper sheets on a copy tray of an inkjet image forming apparatus using a face-down discharging method. Specifically, copy trays in various shapes have been proposed to orderly stack paper sheets thereon. However, even when such copy trays are used for inkjet image forming apparatuses, paper sheets having a high image area proportion such as images having a large size solid image cannot be well stacked thereon if the face-down discharging method is used for the image forming apparatuses.
Although the mechanism of curl of a paper sheet in an inkjet image forming apparatus will be described later in detail by reference to FIGS. 22-24, the surface of the paper sheet, on which ejected ink droplets are adhered, is expanded due to absorption of water included in the ink droplets. Since the paper sheet bearing an ink image thereon is discharged in a face-down manner from the main body of the image forming apparatus, the paper sheet has a U-shape curl such that both the side ends of the paper sheet are higher in level than the central portion thereof.
If the height of both the side ends of the curled paper sheet is greater than the height of a pair of discharging rollers from the bottom of the copy tray (or from the surface of the uppermost paper sheet stacked on the copy tray), a stacking problem occurs in which the curled paper sheet on the copy tray is pushed by the following recording sheet discharged by the pair of discharging rollers (and the paper sheet falls from the copy tray in the worst case), resulting deterioration of stacking quality of the paper sheets on the copy tray. Until now, there has been no proposal for a copy tray designed for inkjet image forming apparatuses using a face-down discharging method, and therefore the stacking problem is not yet solved.
In general, the above-mentioned U-shape curl of a paper sheet has a property such that the height of the curled paper sheet is greatest just after the paper sheet is discharged from a pair of discharging rollers, and the height of the curled paper sheet decreases as time elapses, i.e., as water in the ink image penetrates into the paper sheet. However, the time (hereinafter referred to as decurl time) taken for the paper sheet to have curl not higher than the height of the pair of discharging rollers is generally longer than the copy interval between discharging of the rear end of a paper sheet and start of discharging of the front end of the next paper sheet. When a method in which the copy interval is set so as to be longer than the decurl time is used, the copy speed decreases, resulting in deterioration of the usability and productivity (i.e., performance) of the image forming apparatus.
For these reasons, the inventors recognized that there is a need for an inkjet image forming apparatus which uses a face-down discharging method but does not cause the stacking problem without increasing costs and deteriorating the usability and productivity of the image forming apparatus.